)People exposed to air pollution face greater risk of heart attack, stroke and cardiovascular death, the American Heart Association (AHA) warned on Monday.

The AHA pointed to fine particulate matter as the most evident threat because its tiny size makes it more likely to infiltrate even the smallest airways.

Fine particulate matter, which is called PM2.5 by scientists because the particles have a diameter of 2.5 micrometers or less, is the type of air pollution most likely to cause cardiovascular disease, the AHA said in an updated report on air pollution.

"Particulate matter appears to directly increased risk by triggering events in susceptible individuals within hours to days of an increased level of exposure, even among those who otherwise may have been healthy for years," report lead author Dr. Robert D. Brook said in an AHA news release.

"Growing evidence also shows that longer-term PM 2.5 exposures, such as over a few years, can lead to an even larger increase in these health risks," he added.

As a result, fine particulate matter "should be recognized as a 'modifiable factor' that contributes to cardiovascular morbidity and mortality," the AHA report said.

Those at highest risk from PM2.5 exposure include the elderly, people with existing heart diseases, and possibly those with diabetes, according to the report.

Major sources of PM 2.5 include fossil fuel combustion from industry, traffic and power generation, according to the report.